He shows them the basics: dead man's float, treading water, breast stroke, front crawl. Sam's so buoyant that none of it seems to matter, shifting from one stroke to another as the fancy takes him. It's noisy and messy and under other circumstances John would likely reprimand him for it, but the only purpose of this little trip is to ensure that his boys won't drown themselves in the unlikely chance that they encounter water deep enough to drown in, so he lets Sam have his fun for once.
Meanwhile Dean swims rigid, careful laps, slowly tacking his way up and down the cove, as nearly textbook perfect as he can, given his previous lack of practice. When he reaches the mouth of the cove there's a moment where he hesitates, looking out into the open lake, but then he puts his head down and begins to tack his way towards John and the beach. It's only when Dean's nearly halfway back that John has the passing thought if he'd kept going John wouldn't have had any way of retrieving him.
Lunch is a pack of hotdogs on sticks over the campfire John uses as a demonstration. Fire starting is another skill John expects that his sons will never need to use, but they're here, so he might as well make the most of the opportunity to cover fire as well as water. Sam's hotdogs all wind up completely burnt, but he claims the charring makes it taste better. Dean, once he maps out the temperatures within the fire, is meticulous about managing the whole cooking process--so much so, that John has him do the last couple in the pack to ensure that Sam will get at least one that's more meat than char.
The sun comes out after lunch. The glare off the water feels a bit like knives in John's eyes, but his boys have resumed swimming and he wants them to come away from this with an actual skill--and that means giving them enough time to practice that it'll stick. So he nurses his next-to-last beer on the shore under what little shade is available, calling out the occasional instruction as Dean finally begins to relax into his strokes, calculated precision turning into a more natural instinct for how the water reacts to each motion.
They're both sunburned by the time John tells them it's time to leave, but that's part of the learning experience as well. Or that's what he'll claim later when he tells them to always use sunscreen and Sam asks why he hadn't brought any. Dean's the one who'll be in utter agony, his fairer skin having turned an angry, painful red that'll have him haunting the shower room for damp towels and cold compresses. For now he's just a bright pink that's lost in the brilliant sunset painting all the world in unlikely colors, a silent, still form beside John in the front seat while a loose-limbed Sam snores in the back.
He stays in the car after they arrive back in the bunker, while John carries Sam off to bed, until John comes to see if he's all right. He's staring off into the distance, expression unreadable, when John opens the passenger door. "Are we going to do that again sometime?" he asks after a long moment, tone carefully undemanding. "Sammy had a lot of fun, but his breast stroke still needs work." He looks up at John, eyes smudged with weariness, cheekbones a sun-blasted brick red, so unknowingly fragile that it terrifies John sometimes.
"I'm glad you enjoyed yourself," John says after taking his own moment to phrase things carefully. He rests a hand on Dean's shoulder, which flinches almost imperceptibly at the touch. "But I don't think we'll have the time to visit the reservoir again. I'm going to be very busy now that Warder Johnson is gone and I've taken his place."
"I understand," Dean says, mouth curling down a little but expression free of accusation. "I'll explain things to Sammy so he doesn't ask about it."
And Sam never does, though soon John's soon so occupied with hunting for Mary's killer and making sure nobody notices the many absences from his post that he never thinks twice about the whole affair, other than a vague reassurance when he sees news stories about someone else's children drowning. His own live safely in an underground concrete box, but even they know how to swim.